1. Field of the Invention
The present invention relates to an automotive headlight discharge bulb configured so that a light distribution pattern having a predetermined clear cut-off line is formed by a light intercepting portion and a light distribution control reflector when the automotive headlight discharge bulb is used as a light source of an automotive headlight.
2. Related Art
One type of an automotive headlight having a discharge bulb is provided with a parabolic reflector 8 and a discharge bulb 1 as shown in FIG. 5 inserted as a light source in the parabolic reflector 8. The parabolic reflector 8 is accommodated within a lamp compartment defined by a lamp body and a front cover. Light emitted from the discharge bulb 1 is reflected by the reflector 8 so as to form a predetermined light distribution.
As shown in FIGS. 15, 16, the discharge bulb 1 as a light source is provided with an arc tube main body 2 in which a cylindrical shroud glass tube 5 having a UV cutting off function is fused integrally to an arc tube 3. The arc tube main body 2 is integrally assembled to a synthetic resin insulating base 7 so that the arc tube main body 2 is fixed and held so as to extend forward. To be specific, a rear end portion of the arc tube main body 2 is seized and fixed to a front side of the insulating base 7 via a metallic fixture 6a. A front end portion of the arc tube main body 2 is supported by a lead support 6b extending from the insulating base 7. The lead support 6b also constitutes an energizing path.
The arc tube 3 is formed with a hermetically sealed glass envelope 3a at substantially a longitudinal center portion of the arc tube 3. Both end portions of the hermetically sealed glass envelope 3a is pinch sealed. In the hermetically sealed glass envelope 3a, luminous materials (metal halides and mercury and the like) are sealed in together with a starting rare gas. Electrodes 4, 4 are provided in the hermetically sealed glass envelope 3a in such a manner as to face each other. Lights are emitted through arc discharge occurring between the facing electrodes 4, 4. As shown in FIG. 15, a pair of left and right light intercepting films 5a, 5b, which are designated as pinstripes, are provided on an outer surface of the cylindrical shroud glass tube 5 integrally fused to the arc tube 3 so as to intercept part of light traveling toward an effective reflecting surface 8a of the reflector 8 to thereby form a sharp clear cut-off line. In addition, a metallic light intercepting shade 9, which is fixed to the reflector 8, is provided around the arc tube 3 fixedly inserted in the reflector 8 so as to intercept direct light attempting to travel forward and light attempting to travel toward other than the effective reflecting surface 8a of the reflector 8.
Further, mercury which performs a buffer action is sealed in the hermetically sealed glass envelope 3a. However, mercury is a hazardous material which causes the global environmental pollution. Therefore, in recent years, the development of a mercury-free arc tube has drawn attention in which no mercury is sealed in a hermetically sealed glass envelope, as shown in JP-A-2002-093369.
However, in the mercury-free arc tube, there are following problems.
In general, automotive headlights are configured so that a dip beam is formed by an effective reflecting surface (a multiple reflecting surface) 8a of a reflector 8. The effective reflecting surface 8a is provided on the reflector 8 at least upside of a position where a bulb is disposed. When the effective reflecting surface (a multiple reflecting surface) 8a is designed, a light source image (an arc image which is a discharge light emitting portion of an arc tube) is projected (affixed), as shown in FIG. 17, radially about an elbow portion O of a clear cut-off line. However, arc bending becomes large by such an extent that mercury is not sealed in the hermetically sealed glass envelope 3a, and a dark portion (refer to shaded portions B in FIG. 18) becomes visible in each light source image (arc image) projected along the clear cut-off line. Hence, there is caused a first problem that the clear cut-off line becomes wavy and does not become rectilinear, whereby visibility is deteriorated by such an extent that the clear cut-off line does not become straight.
In addition, arc becomes thin by such an extent that mercury is not sealed in and arc oscillates due to vibrations generated while the vehicle is running, resulting in a second problem that visibility is deteriorated due to the oscillation of light distribution in which the clear cut-off line oscillates vertically. Furthermore, this causes a third problem that the arc spot formed between the distal ends of the electrodes becomes too bright and hence light distribution irregularity becomes noticeable, whereby visibility is deteriorated further by such an extent that the light distribution irregularity occurs.
Note that the first, second and third problems are not inherent only in the mercury-free arc tubes, and it is said that the conventional mercury sealed in arc tubes had more or less the same problems.